The present invention relates generally to power conversion and conditioning and, more particularly, to a rectifier for reducing current harmonics from an unbalanced source.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids designed so as to be extremely efficient in delivering massive amounts of power. To facilitate efficient distribution, power is delivered over long distances as low frequency three-phase AC current.
Despite being distributable efficiently, low frequency AC current is sometimes not suitable for end use in consuming facilities. Thus, prior to end use power delivered by a utility has to be converted to a useable form. To this end a typical power “conditioning” configuration includes an AC-to-DC rectifier that converts the utility AC power to DC across positive and negative DC buses (i.e., across a DC link) and an inverter linked to the DC link that converts the DC power back to three phase AC power having an end-useable form (e.g., three phase relatively high frequency AC voltage). A controller controls the inverter in a manner calculated to provide voltage waveforms required by the consuming facility.
Motors and their associated loads are one type of common inductive load employed at many consuming facilities. While the present invention is applicable to different load types, to simplify this explanation, an exemplary motor with an associated load will be assumed. To drive a motor an inverter includes a plurality of switches that can be controlled to link and delink the positive and negative DC buses to motor supply lines. The linking-delinking sequence causes voltage pulses on the motor supply lines that together define alternating voltage waveforms. When controlled correctly, the waveforms cooperate to generate a rotating magnetic field inside a motor stator core. The magnetic field induces (hence the nomenclature “induction motor”) a field in motor rotor windings. The rotor field is attracted to the rotating stator field and hence the rotor rotates within the stator core.
Standard AC drive topologies utilize AC-DC-AC power conversion with a three phase rectifying bridge for the AC-DC function. A three-phase diode bridge generates pulse type current that includes significant total harmonic current distortion. As AC drives proliferate, equipment system specifications limiting the amount of harmonic current injected into the utility grid are becoming more common and thus solicit cost effective harmonic mitigation solutions. System specifications are often written so measured total harmonic distortion at the Point of Common Coupling (PCC) complies with the maximum low voltage total harmonic distortion levels (THDV) and system classification of IEEE 519. The PCC is usually at the power metering point where other customers connect to the common line voltage but may also be within a plant where linear and non-linear loads are connected. One technique for reducing the harmonics content is to add filters on the AC side of the rectifier. However, the installation of filters can make the overall system bulky and expensive.
A multiple-pulse rectifier is an alternative way to address the distortion problem. Multiple-pulse rectifiers employ a transformer to convert the 3 phase input voltages to a higher number of phases and then use a larger bridge to rectify the waveforms. For example, an 18 pulse rectifier employs a 9-phase transformer to generate 9 voltage signals from a 3 phase source and an 18 diode rectifying bridge to generate the current pulses. Such a multiple-phase rectifier provides low input current THD, near unity power factor under all working conditions, no resonance, and cost effectiveness in higher power ratings. The 18-pulse rectifier is capable of meeting the IEEE-519 specification requirements under balanced source voltage.
However, the 18-pulse rectifier is susceptible to source distortions. Among them, phase unbalance is the most significant condition because the 18-pulse rectifier shows almost zero impedance to the negative sequences. Another significant distortion are the 5th order harmonics commonly seen in power systems using six diode/thyristor rectifiers located near the PCC. The effects of these factors, phase unbalance and harmonic content, increase to certain values, the current THD of an 18-pulse rectifier may increase significantly and exceed the IEEE-519 recommended current limit.